Windows Server 2012 Datacenter - Can run any earlier version of Windows Server Datacenter in addition to being able to run Standard or Enterprise in the virtual instances associated with the Datacenter license

Windows Server 2012 Standard - Can run any earlier version of Windows Server Standard or Enterprise edition (since it's no longer possible to buy Enterprise edition)

Using the downgrade or down-edition rights doesn't mean you can't use the license for Windows Server 2012 on the hardware.